The extent to which British soldiers on any military operation in any country will be able to sue the government under the human rights act will be decided by a landmark case which opened in the supreme court today before nine of the country's most senior judges.

The Ministry of Defence is challenging an appeal court ruling which, it claims, will have a serious impact on commanders by allowing troops to sue even as a result of decisions made in the heat of battle.

Opponents argue the ministry's fears are exaggerated and that soldiers must be protected by the act as "they are subject to UK law wherever they operate in the world".

The case was triggered by the death of Private Jason Smith, 32, of the Territorial Army, who died of heatstroke in Basra in 2003. At the inquest, in 2006, Andrew Walker, the assistant deputy coroner of Oxfordshire, said his death was caused "by a serious failure to recognise and take appropriate steps to address the difficulty that he had in adjusting to the climate".

The inquest heard that Smith had repeatedly told medical staff he was feeling unwell because of the heat – sometimes over 50C – before reporting sick in that August that year. He was found lying face down and taken to hospital, but suffered cardiac arrest.

The coroner said a lack of care amounted to a breach of Smith's human rights.

The MoD agreed in previous court hearings that British soldiers inside a British base are covered by both the protection and the obligations of the act. But the appeal court ruling laid down broader principles. It argued that as British soldiers were subject to British jurisdiction wherever they went, there was no reason why the link between soldier and the UK only existed inside British bases and not outside as well.

James Eadie QC, for the MoD, said the ruling meant the government would have to guarantee human rights protection to soldiers "on duty wherever they are in the world, even though the UK has no executive, legislative and judicial functions in those places or sufficient control of territory". He said it "would impose an obligation upon the UK to be able to ensure that a British soldier on duty a market in Kabul, Afghanistan, can enjoy [European] Convention rights without hindrance, even from those Afghans over whom the UK has no legislative or practical control and where the territory is not controlled by the UK".

He added: "The imposition of some form of legal duty of care would create a major and disproportionate risk that military decision-making would be made more cumbersome and would be skewed in the light of it." He said it could lead to commanders becoming less effective when it came to tactical decision-making and would have an impact on operational effectiveness.

However, Jocelyn Cockburn of law firm Hodge Jones & Allen, representing Smith's mother, Catherine, said "How can it be right for servicemen and women to lose their human rights when they are sent abroad to fight on our behalf? A lot of misinformation and scaremongering has been put out about the implications of this judgment, for instance that it will make our soldiers more vulnerable or hamper military operations. This is absolutely untrue." The Human Rights Act required the government to take reasonable steps to protect the lives of soldiers and a court would not second guess "in any circumstances what a commander decides to do in the heat of battle", she said.

She added: "One can only wonder whether we would be hearing the constant complaints of lack of equipment for service personnel if the government had recognised their 'human rights' from the start. Rather than appealing this case the secretary of state [Bob Ainsworth] should be prioritising the safety of his troops."

The supreme court hearing is listed for three days. The outcome will affect future inquestsinto the deaths of soldiers and how they are conducted.